Clear air turbulence (CAT) is the turbulent movement of air masses in the absence of any visual cues such as clouds, and is caused when bodies of air moving at widely different speeds meet. The atmospheric region most susceptible to CAT is the high troposphere at altitudes of around 7,000-12,000 meters (23,000-39,000 ft) as it meets the tropopause. Here CAT is most frequently encountered in the regions of jet streams. At lower altitudes it may also occur near mountain ranges. Thin cirrus clouds can also indicate a high probability of CAT.
CAT can be hazardous to the comfort, and even safety, of air travel. The thermal characteristics of CAT are known. Studies show that gust velocity changes in CAT of at least 20 ft sec−1 are associated with temperature changes of 3° C. or higher; very few being less than 1° C. Such studies show that CAT horizontal temperature gradients with a minimum temperature change of 2° C., and at a rate which equaled or exceeded 0.5° C. per minute. Moderately choppy CAT was observed at a 5° C. temperature change.
Conventionally, CAT has been measured using active electro-optical heterodyne laser velocimeter systems at ranges exceeding 10 km. Such active systems typically use 10 micron wavelength LWIR (long wavelength infrared) CO2 lasers, larger germanium optics and heterodyning optics. Fast, complex signal and data processing renders systems constructed along these lines are expensive, power-hungry, heavy, and physically large. Further such active systems require much maintenance on a use-by-use basis in alignment, cleaning etc.